Category: 2017

Igneous rock textures encode important information about magma reservoir dynamics. Specifically, the size, shape and abundance of crystals can record multiple phases of crystallisation and magma mixing. However, characterising rock textures using traditional manual methods is extremely time consuming. However, the potential for quantifying textures with automated mineralogical methods, which have seen widespread use in the ore petrology community for some time, has yet to be evaluated.

We investigated samples from across the long-lasting Laki fissure eruption, Iceland, in order determine whether crystal mush occurred at the start of the eruption, or throughout its eight-month duration – an important consideration for understanding magma reservoir dynamics and geometry. We did this by using traditional approaches to determine phase proportions and plagioclase size distribtuions, as well as novel QEMSCAN-based approaches. Although we found significant differences between the manaul and automated datasets, largely because of the inability to easily segment glomerocrysts in the latter, being able to easily combine textural and compositional data was a powerful advantage of the automated approach.

Combined composition-size distributions of plagioclase in samples from the Laki eruption. A0.5 is the square root of crystal area. Figure from Neave et al. (2017).

By fitting high-quality, manually derived plagioclase size distributions, we estimated that mush disaggregation occurred around ten days before the eruption of each sample. These observations, which align well with findings from other stidies (Hartley et al. 2015; 2016), suggest that mush disaggregation was progressive and occurred throughout the eruption: the total volume of eruptable magma active at any given time was much less than the final erupoted volume of 15.1 km3.

At the end of summer 2017, I gave a talk at the annual meeting of the Deutsche Mineralogische Gesellschaft (DMG) at GeoBremen2017. My talk focussed on the results of my 3-kbar experiments on primtive Icelandic basalts, and how they show that depleted mantle melts are much less likely to survive being processed during their ascent through the crust than enriched melts. In other words, enriched melts are more likely to erupt at the surface and depleted melts are more likely to freeze at depth, fundamentally biasing the record of oceanic magmatism we see at the surface. You can download my slides here.

Experimental liquid lines of descent (LLD) for melts from the depleted Háleyjabunga (Hál) and enriched Stapafell (Sta) eruptions. Grey dots show Icelandic compositions from the Western Volcanic Zone and Reykjanes Peninsula (Shorttle & Maclennan, 2011). The arrow shows where compositions were resynthesised to mimic fractional crystallisation.

In summer 2017, I presented a poster at the excellent IAVCEI Scientific Assembly in Portland. My contribution summarised the findings of my experimental work in Hannover so far. In particular, I focussed the effects of mantle-dervied heterogeneity on the phase equilibria of primitive Icelandic basalts in the 1–7 kbar range. You can download a copy of my poster here.

False-colour backscattered electron image of the run products of an experiment on the Háleyjabunga eruption, Iceland

Since moving to Hannover, I have become involved in a number of exciting new projects. One project, lead by François Holtz and carried out by Sören Wilke, involved carrying out a large number of experiments to determine how variations in the anorthite (i.e. calcium) and water contents of rhyolites affects the position of quartz–feldspar cotectics. This is important because the position of quartz–feldspar cotectics can be used as a geobarometer, especially is systems lacking pressure sensitive minerals such as amphibole, but only if the effects of anorthite and water contents are appropriately accounted for.

Our experiments allowed us to define thermal minima and quartz–sanidine–plagioclase triple points on quartz–feldspar cotectics at various pressures, water contents and anorthite contents. This information was then used calibrate an empircal barometer (DEtermination of Rhyolite Pressures; DERP) to esimtate the storage pressure of rhyolitic glasses in equilibrium with quartz and at least one feldspar. DERP is calibrated in the range 50–500 MPa and for any H2O content. Importantly, our findings suggest that rhyolite-MELTS may underestimate the storage pressures of rhyolitic magmas. Bringing emprical and thermodynamic geobarometers into alignment thus represents a key next step in the investigation of rhyolitic magmas.

Thanks to their alkali-rich compositions, pantellerites have much lower viscosities than other rhyolites. As a consequence, these peralkaline magmas erupt in myriad ways to create volcanic landforms that range from lava domes to ignimbrites and fountain-fed lava shields. However, the mechanisms by which such low viscosity melts are able to fragment in explosive eruptions are poorly understood despite the hazards presented by pantellerite volcanoes, above all in the East African Rift.

Building on her MSci project, Ery Hughes investigated this problem of magma fragmentation under the supervision of Marie Edmonds, Kate Dobson and myself by combining 2D (electron microscopy) and 3D (X-ray microtomography) measurements of pumice samples during my MSci.

Slices through an X-ray tomography volume of a pumice from Pantelleria (Hughes et al. 2017).

We found that pantelleritic pumices from Pantelleria are texturally indistinguishable from calc-alkaline pumices from a range of rhyolitic systems, implying that our peralkaline pumices fragmented in a brittle fashion and that their unusual chemistry had little effect on their syn-eruptive textural evolution. We therefore propose that the observed pumice textures developed in response to high decompression rates and that peralkaline rhyolite magmas can fragment when strain localisation and high bubble overpressures develop during rapid ascent.

Geochemcial records of mantle processes are progressively degraded as magmas differentiate and ascend towards the Earth’s surface. This degradation is particularly severe in the case of volatiles (H2O, CO2, F, S and Cl) that decouple from melts upon reaching vapour saturation. Melt inclusions – pools of silicate liquid that are partially insulated from changes in the external magmatic enronment by their host crystals – are thus appealing targets for investigating the behvaiour of magmatic volatiles. Although numerous recent studies have critically evaluated the effects of syn- and post-entrapment modification on olivine-hosted melt inclusion compositions, little comparable information is available for plagioclase-hosted systems, depite plagioclase’s abundance in mafic magmas.

In order to address this imbalance in undertanding between olivine-hosted and plagioclase-hosted systems, we present volatile and light lithophile element analyses from a large number of mainly plagioclase-hosted melt inclusions from the 10 ka Grímsvötn tephra series from Iceland. Major and trace element data have already been presented in study into the pre-eruptive evolution and storage of the tephra series (Neave et al., 2015).

Volatile-trace element systematics in matrix glasses and melt inclusions used to distinguish between pre-, syn- and post-entrapment signals of variability. Figure from Neave et al. (2017).

The uniformly low CO2 content of melt inclusions cannot be explained by either shallow entrapment or shrinkage bubble formation, suggesting that inclusion CO2 contents were controlled by decrepitation instead. High H2O/Ce values in primitive plagioclase-hosted inclusions (182–823) are most easily accounted for by diffusive H2O gain following the entrainment of primitive macrocrysts into H2O-rich melts a few days before eruption (e.g., Hartley et al., 2015). Extreme F enrichments in primitive plagioclase-hosted inclusions (F/Nd = 51–216 versus 15 in matrix glasses) possibly reflect the entrapment of inclusions from high-Al/(Al+Si) melt pools formed by dissolution-crystallisation processes (as indicated by HFSE depletions in some inclusions), and into which F was concentrated by uphill diffusion: F is highly soluble in Al-rich melts. The high S/Dy of inclusions (300) indicates that primary melts were rich in S in comparison with most oceanic basalts. Although primitive plagioclase-hosted melt inclusions from the 10 ka Grímsvötn tephra series record few primary signals in their volatile element contents they nevertheless record information about crustal magma processing that is absent from olivine-hosted melt inclusions suites.

Pressure is one of the key intensive variables that controls magmatic phase equilibria, which thus raises the possibily of using mineral and melt compositions to estimate magma storage pressures from erupted products. Such estimations are crucial for addressing geological problems ranging from understanding crustal accretion through to interpreting signals of unrest at active volcanoes.

In this paper with Keith Putirka, I assess the performance of some commonly used barometers that exploit the pressure-sensitive incorporation of jadeite (Jd) into clinopyroxene. We find that many current barometers overestimate the pressure of phase equilibria experiments carried out on H2O-poor basalts at 1–7 kbar by up to 3 bar. Many published magma storage pressure estimates may thus need to be re-evaluated, and revised towards lower pressures.

In order to resolve the of barometer inaccuracy at low pressures, we thus present a newly calibrated Jd-in-clinopyroxene. Our new barometer is suitable for use on hydrous and anhydrous samples that are ultramafic to intermediate in composition. However, we do not recommend using the barometer at temperatures below 1100 °C and at oxygen fugacities above QFM+1 because of reduced accuracy under these conditions. The barometer reproduces its calibration data with a standard error of estimate (SEE) of 1.4 kbar, and tests performed using experiments on bastilc compositons confirm that it is significantly more accurate than previous models.

Calibration and test datasets used to develop and assess a new Jd-in-clinopyroxene barometer for use on ultramafic to intermediate compositions.